Temperature compensated crystal oscillators (TCXO) circuits are known to comprise an oscillator circuit and a temperature compensation circuit. The oscillator circuit includes a piezoelectric crystal and may be one of a variety of known oscillator topologies, such as Colpitts, Pierce, or Hartley oscillators. Regardless of the chosen oscillator circuit topology, the oscillator circuit ideally generates a very narrowband alternating current (AC) signal at a predetermined center frequency. Thus, the oscillator circuit utilizes the crystal to provide a tuned, resonant circuit with a high quality factor (Q) at the oscillator circuit's operating frequency. The crystal's high Q accommodates the desired narrowband frequency response of the oscillator, a frequency response that cannot be achieved using lower Q, discrete inductors.
As is also known, the crystal is a circuit element whose frequency characteristics are responsive to variations in ambient temperature. For example, the crystal's resonant frequency depends on the physical shape, blank orientation (i.e., cut type, and angle), elasticity constant, and density of the piezoelectric material. Since these characteristics of the piezoelectric material vary during ambient temperature excursions, a corresponding change in the crystal's resonant frequency results.
In an attempt to limit the variation of the crystal's resonant frequency over temperature, crystal cuts are generally selected to provide a substantially flat frequency-versus-temperature response over a desired temperature range. An AT-cut crystal, which has a third order frequency versus temperature characteristic, typically provides a flat frequency versus temperature response over a temperature range of -30.degree. C. to +85.degree. C. when compensated. Similarly, GT-cut crystals might be used to provide a flat frequency versus temperature response over a more limited temperature range (e.g., from -10.degree. C. to +50.degree. C.).
Since the crystal's resonant frequency varies as a function of temperature, the oscillator's output signal frequency also changes as a function of temperature. Over a temperature range of -30.degree. C. to +85.degree. C., the oscillator's output signal frequency may change approximately +/-20 parts per million (ppm) for a typical AT-cut crystal when no compensation has been added to the oscillator. For example, an uncompensated oscillator circuit using an AT-cut crystal with a 10 MHz resonant frequency can be expected to vary in output signal frequency by as much as 200 Hz.
To reduce the oscillator frequency variation, the TCXO utilizes the temperature compensation circuit to correct for frequency deviations due to variations in temperature. Temperature compensation of the oscillator circuit is typically achieved by placing a temperature dependent reactance in series with the crystal. The temperature dependent reactance is selected to cancel the variation in crystal reactance caused by changes in ambient temperature. The temperature dependent reactance typically comprises a varactor diode whose applied voltage is determined by the temperature compensation circuit, depending on the amount of compensation reactance necessary. With this temperature compensation, the oscillator's output signal frequency remains substantially constant over variations in ambient temperature, thus resulting in a substantially flat frequency-versus-temperature response for the oscillator circuit. For example, the output signal frequency variation of the aforementioned 10 MHz oscillator circuit may improve from +/-20 ppm to +/-1 ppm (i.e., 10 Hz) with temperature compensation.
In addition to temperature effects, the crystal's frequency varies over time due to aging. Aging typically results from stress relief in the crystal or mass transfer to, or from, the crystal. To correct for this phenomenon, the oscillator circuit is adjusted to counteract, or offset, the aging. Aging correction is generally accomplished by varying the oscillator's load reactance by a predetermined amount, thus forcing the oscillator circuit to operate at a new load reactance point. Operation into the new load reactance inherently impacts the frequency response of the oscillator circuit, and results in correcting the aged oscillator's output signal frequency. A varactor, variable capacitor, or variable inductor is typically used to provide the aging adjustment. The aging adjustment is substantially independent of the temperature compensation circuit.
By providing the aging adjustment and temperature compensation of the TCXO independently, a result known as the trim effect occurs. Since crystal aging is corrected by changing the oscillator circuit's load reactance (i.e., the load capacitance of the crystal), the temperature compensated oscillator circuit operates into a different load after an aging adjustment. Changes in the crystal's load capacitance affect the crystal's frequency sensitivity and, correspondingly, the amount of compensation provided by the temperature compensation circuit.
The crystal's frequency sensitivity is given by the following equation: EQU crystal sensitivity=(.omega.C.sub.m)/{2 [(C.sub.0 /C.sub.1)+1].sup.2 }.times.10.sup.6 [ppm/ohm]
where C.sub.m =crystal motional capacitance;
C.sub.0 =static capacitance of the crystal; PA1 C.sub.1 =load capacitance; and PA1 .omega.=angular frequency.
Thus, changes in the crystal's load capacitance (C.sub.1) during an aging adjustment disrupts the substantially flat frequency versus temperature response produced by the temperature compensation circuit. This disruption is known as the trim effect. The trim effect is not a serious problem with low stability oscillators (e.g., greater than 5 ppm); however, with the growing demand for high stability oscillators (e.g., 1 ppm or less), the trim effect has a noticeable effect on oscillator stability performance.
Prior art techniques attempt to solve the trim effect dilemma in two distinct manners. The first technique is to neutralize the crystal's static capacitance (C.sub.0), thus creating a constant crystal sensitivity as a function of load capacitance. A crystal's static capacitance is typically neutralized by placing an inductor in parallel with the crystal. This technique generally results in an oscillator circuit that is more prone to generating undesirable spurious responses due to the increased number of feedback paths produced within the oscillator circuit by incorporating the parallel inductor. In addition, this technique is impractical for fundamental-mode crystals since unreasonably large inductances are typically required. The second technique is to temperature compensate the oscillator's output signal frequency by using compensation external to the TCXO circuit, such as adding or deleting bits from the output signal waveform. While this compensation technique can provide excellent performance when used in dual mode oscillators, such as those described in U.S. Pat. Nos. 4,160,183 and 4,872,765, it is a fairly complex and expensive procedure that is not suitable for moderately priced products.
Therefore, a need exists for a method and apparatus for reducing the trim effect of a TCXO circuit that is not constrained by the shortcomings of the prior art. In particular, a compensating technique that can be used over a wide range of operating parameters and that uses a desired aging adjust without requiting external compensation circuitry would be an improvement over the prior art.